Apache Camel : "direct:start" endpoint - what does it mean?

I'm new to Apache Camel. Can someone explain what "direct:start" means in Camel. Please see

https://camel.apache.org/components/latest/http-component.html

from("direct:start")
.to("http://myhost/mypath");

Thanks.

The "direct:start" above is simply saying that the route starts with a Direct Component named "start".

The direct endpoint provides synchronous invocation of a route. If you want to send an Exchange to the direct:start endpoint you would create a ProducerTemplate and use the various send methods.

ProducerTemplate template = context.createProducerTemplate();

template.sendBody("direct:start", "This is a test message");

There is nothing special about the name start. It is simply the name you are going to use when referring to the endpoint and could have just as easily been direct:foo.

Assume like the direct route as a method with name start , so we need to call the start method /direct route to perform certain operation. The below example will help .

The first route will be triggered when an input file is available in XXXX location and when it reaches line , the actual flow will go to second route. Basically the direct route with from endpoint will be triggered by some producer endpoint.

<route id="fileRoute">
   <from uri="file:XXXX">
      ..
   <to uri="direct:start">
</route>

<route id="directStartRoute">
    <from uri="direct:start">
    <to uri="http://myhost/mypath">
</route>

Apache Camel direct is basically for sending Exchange from one route to another in SAME Camel context. So let’s say you are getting message from AMQ and you want to populate headers for every message you get and then send it to mail recipient list. So here you need to create new router which has following description

from(“direct:populateHeaders”)
.setHeader(“myHeader”, “myHeaderValue”)
.end()

And from any route you can send your Exchange object to this route by writing

...

.to(“direct:populateHeaders”)

...

Its important to keep in mind that this will not work out of your Camel Context.

direct:start provides synchronous ways to communicate between 2 endpoints and this is only been used if you want to communicate using camel messages and not generic file or xml messages.

Consider it like this : There are two things whenever you are sending a message to camel route.

1. The URI scheme, which defines how your message is going to be delivered. And to which component type it is going to be delivered.
2. URI path, which defines the instance of that component.

Now, to your direct:start location. 'direct' tells that this message should send synchronously to the Direct Component. 'start' tells which instance of the Direct Component this message should be delivered.

Importance of different URI path: Now consider if you are having to different routes. And wants to produce message from two different threads synchronously. Using 'direct:start' as start point for the routes will not work. Unless you are having some conditional processing component, forget this if you are beginner. For, successfully deliver the messages to both the routes , you will have to add 2 entries i.e. 'direct:somename1' and 'direct:somename2'. 'start' is not a mandatory thing , you can give whatever name you like to.

I recommend you to read some chapters from Martin Fowler's EIP books. It is a wonderful resource to begin with. This will make you very easy to understand Camel.

It should be fairly easy to explain: exchange is always sent from a source to a destination. For that to happen, you need to create those 02 endpoints: one will consume (yes the start) and the other one will emit. from("direct:start") means "directly consume the exchange from the "start" endpoint and process it anyhow. to("direct:start") means "send" the exchange to the "start" endpoint "directly" within the same camel context. What makes this really ambiguous is that the endpoint itself (i.e: "direct:start") is implicitly created on the fly so when writing your code, you are assuming that there is an endpoint called "direct:start" so you can retrieve exchange from it but you can also send an exchange to it. Good luck!

forget about start. start is just a name of a halt point (direct).

Direct component can be take as a bridge between routes in same context.